Strain measuring circuit



May 19, 1953 F. L. HOLF'ORD 2,638,730

STRAIN MEASURING CIRCUIT Filed April 5, 1948 I I i l g V1 l6 u I7 H- 1 /I2.

n IO l II Fran/r orci 1 /514,

Patented May 19, 1953 UNITED STATES PATENT OFFICE STRAIN MEASURING omoUr-r Frank L. Holford, Towson, Md., assignor to The Black. & Decker M nu acturing Comp ny, a corporation of Maryland Application April 5, 1943-, Serial No. 18,889

l Clai s- -1 The present invention relates to a circuit for measuring the deflection of a magnetic strain gage. The magnetic coil or coils of the gage are attached to the member, such as a scale, on which an object is being weighed and they have an armature near them which moves with the variations of weight on the scale and thereby changes in the magnetic flux of the coil or coils take place which are in proportion to the weight placed upon the scale.

Among the objects of the present invention are to provide a circuit having high sensitivity and high stability.

Another object is to provide a circuit in which there is little or no distortion of the metered readings arising from phasing out a large residual alternating current bridge output voltage due to unmatched voltage phases in the parallel legs of the bridge at varying conditions of apparent balance. Another object of the invention is to eliminate the eifects of rectifier drift upon the metered readings at balance. Another object of the invention is the elimination of the effects of non-linear rectifier characteristics upon the metered readings at balance. Another object of the invention is the elimination of the efiects of unmatched rectifiers upon the metered readings at balance.

Another object of the invention is the compensation for the curvature of voltage differential as a function or the magnetic strain gage movement arising from the basic behavior of that gage as applied'to the particular scale in question, in order to obtain linear metering results when balancing the strain gage bridge with a linearly caiibrated potentiometer.

In meeting the above objects it is essential to have economy and simplicity for manufacturand for calibrating purposes and the components must be rugged and insensitive and possess a latitude of operating tolerances which except for the strain gage and calibrated potentiometer, can be met by standard manufactured stock parts.

The above and other objects and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings forming a part hereof and in which:

Figure 1 illustrates a circuit embodying the principles of this invention.

Figure 2 is a vector diagram of characteristics of branch 4, in relation to the phase angle oi"; the current It flowing in that branch.

Figure 3 is. a vector diagram of characteristifls to e 3- o branch H), n relat on to the pha n le of the current In; flowing in that branch.

Figure i is. a sectional view of part, of a scale showing the inductors or inductive reactances of the instant. in en on- The circuit is illustrated in connection with the scale shown in Figure 3,5 of the application of James N. Baker filed January 10., 1945, Serial No. 572,179. Further details of the operation of the scale proper will be apparent in that application.

As shown in the drawings two electro-magnetic coils having cores therein, l and 2 are placed ne ab and h il r b n ath a m vin r m The destro -magnetic coils or inductors are fixed in the scale the armature 3 moves in dir ct pr portion to the load up n t c e, so that the mag etic flux of th two coils varies as nearly as possible dineot. proportion to the load on the scale it is to the measurement f his vari bl flu hat th instant i v nti n is. direc ed. The coils qr in cto and 2 a howin F gure l. as be ng op o it es o an 4 of. a rid c cu t n rgized iro a al ernati g c r-rent sourc E t a d i t e ranch 4 brid The other components of this branch ii! are th Phase com n at n cond er 3 an 14, the zero setting variable resistances l4 and I5 ange c nn cted to eth r in oppos t that s, s that when t esi ta e M s in reased the resistance 55 i simu taneous rea a d vi ve sa T i b anch a so nclude t var ab e es stance 5 nd l wh c ar s ns or co necied. o ther in tandem t a is o a h is i creased t e othe is mul a us increased. The purpose of these latter resistances is to calibrate the bridge circuit so that the ad n -s on t otentiom ter will m a u e t han es flux th i du tors n u its f physical forceor pounds- The zero setti re i t nc s are to. adj st the two branches .6 a d. H to bail ante when the sc le and th poten te a e t ze o. The note meter arm 42 onstitutes the other output; to inal of the brid e c nsist.-

- ing of the two branches 4 and i0.

The alternating current source 5-6 passes through additional resistance 18 to the primary coils or windings I9 of isolation transformer 20. The secondary coils or windings M of this transformer place what may be called a standing or polarizing alternating current, of which an instantaneous value is shown to be moving in the direction of the arrows 39 and 40, upon a double balance indicating circuit 22-23 and which may be connected by double switch 2425 with the output terminals of the bridge circuit I2 and 9 respectively. The switch 2425 is intended to be closed for initially balancing the bridge circuit and to take a reading of weight.

The standing current on this double balance indicating circuit 2223 maintained by the transformer 20 through its secondary coil 2| forming the third and common conductor for this double circuit, passes through two full wave rectifiers 26 and 2's. These two rectifiers are connected in series on their direct current side by conductors 28 and 29 and midway between each of these conductors, points of equal potential, galvanometer 38 is connected. Conductor 29 connecting the galvanometer with rectifier 26 has fixed resistance 3| therein and the same conductor connecting the galvanometer with rectifier 21 has variable resistance 32 therein. The two rectifiers 26 and 21 are arranged in parallel with the secondary 2i of the transformer 20 as shown. The standing or exciting current in the circuit 2223 is held to a very low value by the high resistance ill in the primary of the transformer and by resistances 33 and 34 in the respective branches of the indicating circuit 2223 in the secondary. Because of this very low value vagarious changes in its magnitude become insignificant relative to the bridge output voltages subsequently impressed upon it.

The galvanometer 30 is kept at zero reading with the switches 24 and 25 open by adjustment of the variable resistance 32. After this adjustment to balance is made any unbalanced voltage impressed upon the circuit 2 I, 22 and 23 from the output terminals 9 and I2 of the bridge will cause a current to flow in this double balance indicating circuit, of which current an instantaneous value is indicated in the direction of the arrows 35, 36 for one condition of unbalance. This current adds to the current in resistance 33 and increases the voltage on rectifier 25 and subtracts from the current in resistance 34 and decreases the voltage on rectifier 21. The galvanometer connections with conductors 28 and 29 are no longer points of equal potential and the galvanometer will swing off zero in one direction. If the arm I2 of the potentiometer is adjusted to the opposite side of balance, after the bridge circuit was adjusted to zero position by the zero setting resistances I t and I5 with the armature 3 being in position of no load, thereby reversing the voltage between output'terminals 9 and I2 with respect to the standing current on the circuit EL 22 and 23, the change of potential through the galvanometer will be in the opposite direction, and the galvanometer will swing in the opposite direction. When the bridge is balanced and the voltage between the output terminals 9 and I2 is zero, or when the residual voltage at these terminals is 90 out of phase with the standing excitation of the circuit 2|, 22, 23 there will be no change in the values, or relative values, of the standing current, and the galvanometer will give no indication in either direction.

Figures 2 and 3, the relative phase angles of the voltages across the components of the branches 4 and III are shown. In Figure 2 the voltage phase I4 I42 is seen to lead the supply voltage V4 by an angle M depending upon the relative fixed resistance values of the inductors or inductance coils I and 2, while the interchange of voltage between these two coils as the strain gage is moved corresponds to a moving point A vertically upon the total inductive-reactance vector. Similarily, see Figure 3, when fixed condensers are added to branch I0, the voltage phase across the potentiometer and the other total resistances, R is seen to lead the supply voltage V10 by an angle N, depending upon the value of the condensers added. Adjusting the potentiometer corresponds to moving point B horizontally upon the resistance vector.

The supply voltage V4 and V10 for each diagram is the same voltage and if the vector diagram of Figure 3 be rotated and superimposed upon its equivalent Figure 2, then it will be seen that the two diagrams may be made'to coincide exactly throughout, so that points A and B will exactly coincide for all positions of balance if the correct condenser values are chosen. There will therefore be no residual voltage between output terminals 9 and I2 and true balance will occur.

The magnetic strain gage above described superimposes upon a straight line relationship between voltage interchange between coils and dimensional displacement a concave curve of increasing voltage interchange for each interval of displacement. This curve arises from a combination of uncontrolled air gap loss, fringing at the air gaps, permeability curves for iron, and inductance-resistance ratios of the coils. This is compensated for by selecting resistances 31 and 38 and adding them to the circuit between zero setting resistances I4 and I5 and potentiometer arm I2. These resistances have their value determined by experiment, they may be termed bleeder resistances, and their action is to add a pair of concave reciprocal curves to the straight line behavior of the linearly calibrated metering potentiometer, effecting a straight line relationship between gage displacement and potentiometer adjustment. This modification obviates the need for a calibrated non-linear dial in connection with the linear potentiometer.

In operation, with the switches 24 and 25 open the circuits 2|, 22 and 23 are balanced through movement of the variable resistance 32 and these circuits may always be tested for balance in this manner. The bridge circuit in the position of no load may always similarly be tested for balance by closing the switches 24 and 25 and with the potentiometer arm I2 in the zero position, by adjusting the zero setting resistances I4 and I5. With the bridge circuit thus balanced the load is placed upon the scale and with the switches 24 and 25 closed the reading of the galvanometer 30 is again brought back to balance by movements of the potentiometer arm I2 in the required direction. The weight of the load is thereupon determined by the displacement of the potentiometer arm [2 from its zero position. And if it is desired to again check the balance of the metering circuit 2 I, 22 and 23 this may be done by opening the switches 24 and 25.

It will be seen that high sensitivity of the circuit is provided because the output voltage of the bridge is impressed upon an indicating circuit in which its eifect operates upon the two full wave rectifiers to increase the output of one and decrease the output of the other, doubly destroying a pre-existing balanced condition.

High stability is provided by holding the standing currents in the indicating circuits to the minimum operable value and by providing continuously adjustable balance of the indicating circuit when the switches 24 and 25 are open.

Distortion due to phasing-out is eliminated as above described in connection with the condensers l3 and I4.

Rectifier drift can have no effect upon the indication of balance since the rectifiers operate in an isolated indicating circuit which is continuously adjustable independently of the bridge circuit.

Non-linear rectifier characteristics can have no effect upon the indicated point of bridge balance because the rectifiers operate at constant excitation level for all positions of balance, and no utilization of rectifier output functions to determine the magnitude of unbalance.

Unmatched rectifiers can have no effect upon the indication of balance, as no metering or measuring of rectifier output is attempted.

Compensation for the non-linear behavior of the magnetic strain gage is accomplished by the addition of the resistances 3i and 38 as above described.

Having described the invention in its preferred embodiment, it is of course understood that it is not limited to the precise construction shown, but may be modified by one skilled in the art.

What is claimed as new and is desired to be secured by Letters Patent is:

1. A strain age circuit comprising a bridge circuit, having two output terminals, a pair of inductors whose relative inductance values are varied in accordance with the strain to be measurcd in one branch of said circuit, one on each side of one output terminal of said circuit, a source of alternating current for energizing said bridge circuit, a sliding arm potentiometer in the other branch of said circuit, comprising the other two legs of said circuit, the sliding arm of which constitutes the other output terminal of said circuit, a two branch balanced indicating circuit comprising a common conductor and two branches connected in parallel across the common conductor, a transformer energizing the common conductor in the balanced indicating circuit from the source of alternating current, a rectifier bridge in each branch of the balanced indicating circuit having their output terminals connected in series and a current responsive instrument connected between their respective output terminals, means to impress an added current on said two branch balanced circuit when said. bridge circuit is unbalanced comprising a pair of impedance elements, one in each branch of the indicating circuit, said elements being series connected across the output terminals of said bridge circuit to indicate a change in the values of the inductors when the gage is placed under strain.

2. The circuit of claim 1 in which the bridge 6 circuit has between each conductor of the source of alterna n cu rent and achinduotcr a fixed resistance, and between each said conductor and the potentiometer, a zero setting resistance, the two zero setting resistances being ganged in opposition.

3. The circuit of claim 1 in which the bridge circuit has between each conductor of the source of alternating current and the potentiometer a phase compensating condenser.

4. The circuit of claim 1 in which the gage has non-linear behavior characteristics, and in addition, the circuit includes two separate resistances, one placed on each side oi the poteniometer arm, between the arm and the connection of the potentiometer resistance to the source of alternating current, to correct the non-linear behavior of the gage.

5. The circuit of claim 1 in which two resistances are placed in the direct current circuit from the rectifiers, one on each side of the connections to the current responsive instrument, one of said resistances being variable in order to balance the direct current circuit.

'6. A strain gage circuit comprising a bridge circuit having two output terminals, a pair of inductors whose relative inductance values are varied in accordance with the strain to be measured in one branch of said circuit, one on each side of one output terminal of said circuit, a source of alternating current for energizing said bridge circuit, a sliding arm potentiometer in the other branch of said circuit, comprising the other two legs of said circuit, the sliding arm of which constitutes the other output terminal of said circult, means, other than the potentiometer, to balance the bridge circuit, a two branch balanced indicating circuit comprising a common conduc tor and two branches connected in parallel across the common conductor, a transformer energizing the common conductor in the balanced indicat.- ing circuit from the source of alternating current, a rectifier bridge in each branch of the balanced indicating circuit having their output terminals connected in series and a current responsive instrument connected between their respective output terminals, means to impress an added current on said two branched balanced circuit when said bridge circuit is unbalanced comprising a pair of impedance elements, one in each branch of the indicating circuit, said elements being series connected across the output terminals of said bridge circuit to indicate a change in the values of the inductors when the gage is placed under strain, whereby the change in values of the inductors may be determined on :the potentiometer bybalancing the bridge circuit by move ments of the potentiometer arm with the added current impressed on the two branch balanced circuit.

'7. The circuit of claim 6 in which the bridge circuit has between each conductor of the source of alternating current and the potentiometer a variable resistance to calibrate the potentiomstar to show changes in values of the inductors in units of physical force, the two resistances being ganged in tandem.

8. A strain gage circuit in which the gage has non-linear behavior characteristics comprising a bridge circuit having two output terminals, a pair of inductors whose relative inductance values are varied in accordance with the strain to be measured in one branch .of said circuit, one ,on each side of one output terminal of said circuit, a source of alternating current Ior ener- 7 gizing said bridge circuit, a sliding arm potentiometer in the other branch of said circuit comprising the other two legs of said circuit, the

sliding arm of which constitutes the other output terminal of said circuit, a current responsive instrument, a transformer having its primary windings connected to the alternating current source, an indicating circuit, comprising two branch circuits including two conductors connectible respectively with the two output terminals of the bridge circuit, a rectifier bridge in each branch circuit, the transformer secondary windings being connected to both of said conductors and between said rectifier bridges to constitute a common conductor for the two branch circuits, the output terminals of the rectifier bridges being connected in series and the current responsive instrument being connected between their respective output terminals, impedance elements between said secondary windings and each of their said connections to the said two conductors, means to impress the current from the output terminals of the bridge circuit upon said two conductors of the indicating circuit to indicate the unbalance of said bridge circuit, two separate resistances, one placed on each side of the potentiometer arm, between the arm and the connection of the potentiometer resistance to the source of alternating current, to correct the nonlinear behavoir of the gage.

9. A strain gage circuit comprising a bridge circuit having two output terminals, a pair of inductors whose relative inductance values are varied in accordance with the strain to be measured in one branch of said circuit, one on each side of one output terminal of said circuit, a source of alternating current for energizing said bridge'circuit, a sliding arm potentiometer in the other branch of said circuit comprising the other two legs of said circuit, the sliding arm of-which constitutes the other output terminal of said circuit, a current responsive instrument, a transformer'having its primary windings connected to the alternating current source, an indicating circuit, comprising two branch circuits including two conductors connectible respectively with the two output terminals of the bridge circuit, a rectifier bridge in each branch circuit, the transformer secondary windings being connected to both of said conductors and between said rectifier bridges to constitute a common conductor for the two branch circuits, the output terminals of the rectifier bridges being connected in series and the current responsive instrument being connected between their respective output terminals, impedance elements between said secondary windings and each of their said connections to the said two conductors, means to impress the current from the output terminals of the bridge circuit upon said two conductors of the indicating circuit to indicate the unbalance of said bridge circuit, means other than the potentiometer to adjust the bridge circuit'to show a balanced current through the instrument, means to adjust the indicating circuit to show a balanced current through the instrument when the bridge circuit is disconnected therefrom, whereby the strain on the gage may be measured on the potentiometer, by returning the instrument to zero reading by adjustment of the potentiometer arm.

10. The circuit of claim 9 in which the bridge circuit has between each conductor of the source of alternating current and each inductor, a fixed resistance, and between each said conductor and the potentiometer, a zero setting resistance, the two zero setting resistances being ganged in opposition.

11. The circuit of claim 9 in which the bridge circuit has between each conductor of the source of alternating current and the potentiometer a variable resistance to calibrate the potentiometer to show changes in values of the inductors in units of physical force, the two resistances being ganged in tandem. i

12. The circuit of claim 9 in which the bridge circuit has between each conductor of the source of alternating current and the potentiometer a phase compensating condenser.

13. A strain gage circuit in which the gage has non-linear behavior characteristics comprising a bridge circuit having two output terminals, a pair of inductors whose relative inductance values are varied in accordance with the strain to be measured in one branch of said circuit, one on each side of one output terminal of said circuit, a source of alternating current for energizing said bridge circuit, a sliding arm potentiometer in the other branch of said circuit comprising the other two legs of said circuit, the sliding arm of which constitutes the other output terminal of said circuit, a current responsive instrument, a transformer having its primary windings connected to the alternating current source, an indicating circuit comprising two branch circuits including two conductors connectible respectively with the two output terminals of the bridge circuit, a rectifier bridge in each branch circuit, the transformer secondary windings being connected to both of said conductors and between said rectifier bridges to constitute a common conductor for the two branch circuits, the output terminals of the rectifier bridges being connected in series and the current responsive instrument being connected between their respective output terminals, impedance elements between said secondary windings and each of their said connections to the said two conductors, means to impress the current from the output terminals of the bridge circuit upon said two conductors of the indicating circuit to indicate the unbalance of said bridge circuit, means other than the potentiometer to adjust the bridge circuit to show a balanced current through the instrument, means to adjust the indicating circuit to show a balanced current through the instrument when the bridge circuit is disconnected therefrom, whereby the strain on the gage may be measured on the potentiometer, by returning the instrument to zero reading by adjustment of the potentiometer arm, two separate resistances, one placed on each side of the potentiometer arm, between the arm and the connec tion of the potentiometer resistance to the source of alternating current, to correct the non-linear behavior the gage.

14. A strain gage circuit comprising a bridge circuit having two output terminals, a pair of inductors whose relative inductance values are varied in accordance with the strain to be measured in one branch of said circuit, one on each side of one output terminal of said circuit, a source of alternating current for energizing said bridge circuit, a sliding arm potentiometer in the other branch of said circuit, comprising the other two legs of said circuit, the sliding arm of which constitutes the other output terminal of said circuit, a two branch indicating circuit comprising a common conductor and two branches connected in parallel across the common conductor,

a transformer energizing the common conductor in the indicating circuit from the source of alternating current, a rectifier bridge in each branch of the indicating circuit having their output terminals connected in series and a current responsive instrument connected between their respective output terminals, two resistances in the direct current circuit from the rectifiers, one on each side of the connections to the current responsive instrument, one of said resistances being variable in order to balance the direct current circuit, means to adjust the indicating circuit to show a balanced current through the instrument when the bridge circuit is disconnected therefrom said thereby to balance both branches of the indicating circuit, means to impress an added current on said two branch balanced circuit when said bridge circuit is unbalanced comprising a pair of impedance elements, one in each branch of the indicating circuit, said elements being series connected across the output terminals of said bridge circuit to indicate a change in the values of the inductors, and means other than the potentiometer to adjust the bridge circuit to show 10 a balanced current through the instrument, whereby the strain on the gage may be measured on the potentiometer, by returning the instrument to zero reading by adjustment of the potentiometer arm.

FRANK L. HOLFORD.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Publication entitled Strain Gages, in Electronics, December 1943, pages 106-111 and 192. 

